The present invention relates to a logic circuit design and, more particularly, to a logic circuit design for combinatorial and asynchronous logic circuits.
A large body of research has been performed to develop and improve traditional Complementary Metal Oxide Semiconductor (CMOS) techniques for the production of integrated circuits (ICs). The object of this research is to develop a faster, lower power, and reduced area alternative to standard CMOS logic circuits (see A. P. Chandrakasan, S. Sheng, R. W. Brodersen, “Low-Power CMOS Digital Design”, IEEE Journal of Solid-State Circuits, vol. 27, no. 4, pp. 473-484, April 1992, and in A. P. Chandrakasan, R. W. Brodersen, “Minimizing Power Consumption in Digital CMOS Circuits”, Proceedings of the IEEE, vol. 83, no. 4, pp. 498-523, April 1995.) This research has resulted in the development of many logic design techniques during the last two decades. One popular alternative to CMOS is pass-transistor logic (PTL).
Formal methods for deriving pass-transistor logic are known for Negative-channel Metal Oxide Semiconductor (NMOS) transistors. The logic circuits resulting from these known methods yield an NMOS PTL logic circuit having a set of control signals applied to the gates of NMOS transistors, and a set of data signals applied to the sources of the n-transistors. Many PTL circuit implementations have been proposed in the literature (see also W. Al-Assadi, A. P. Jayasumana and Y. K. Malaiya, “Pass-transistor logic design”, International Journal of Electronics, 1991, vol. 70, no. 4, pp. 739-749, K. Yano, Y. Sasaki, K. Rikino, K. Seki. “Top-Down Pass-Transistor Logic Design”, IEEE Journal of Solid-State Circuits, vol. 31, no. 6, pp. 792-803, June 1996, R. Zimmermann, W. Fichtner, “Low-Power Logic Styles: CMOS Versus Pass-Transistor Logic”, IEEE Journal of Solid-State Circuits, vol. 32, no. 7, pp. 1079-1090, June 1997, and K. Bernstein, L. M. Carrig, C. M. Durham and P. A. Hansen, “High Speed CMOS Design Styles”, Kluwer Academic Press, 1998, and K. Bernstein, L. M. Carrig, C. M. Durham and P. A. Hansen, “High Speed CMOS Design Styles”, Kluwer Academic Press, 1998).
Some of the main advantages of PTL over standard CMOS design are: high speed due to the small node capacitances; low power dissipation as a result of the reduced number of transistors; and lower interconnection effects due to a small area.
Most PTL implementations, however, have two basic problems. First, the threshold drop across the single-channel pass transistors results in reduced current drive and hence slower operation at reduced supply voltages. This drop is particularly important for low power design since it is desirable to operate at the lowest possible voltage level. Second, since the input voltage for a high logic level at the regenerative inverters is not VDD, the PMOS device in the inverter is not fully turned off, and hence direct-path static power dissipation can be significant.
There are many PTL techniques that attempt to solve the problems mentioned above. Some of them are: Transmission Gate CMOS (TG), Complementary Pass-transistor Logic (CPL), and Double Pass-transistor Logic (DPL). TG uses transmission gate logic to realize complex logic functions using a small number of complementary transistors. TG solves the problem of low logic level swing by using PMOS as well as NMOS transistors. CPL features complementary inputs/outputs using NMOS pass-transistor logic with CMOS output inverters. CPL's most important feature is the small stack height and the internal node low swing, which contribute to lowering the power consumption. The CPL technique suffers from static power consumption due to the low swing at the gates of the output inverters. To lower the power consumption of CPL circuits, latched complementary pass-transistor logic (LCPL) and swing restored pass-transistor logic (SRPL) circuit styles are used. These styles contain PMOS restoration transistors or cross-coupled inverters respectively. DPL uses complementary transistors to keep full swing operation and reduce the DC power consumption, eliminating the need for restoration circuitry. One disadvantage of DPL is the large area required by the presence of PMOS transistors.
An additional problem of existing PTL is the top-down logic design complexity, which prevents the pass-transistors from capturing a major role in real logic large-scale integration technology (LSI). One of the main reasons for this is that no simple and universal cell library is available for PTL based design. Not all variations of input values to a basic PTL cell produce well-defined logic values. This creates difficulties in the development of automatic design systems for PTL logic, and in the verification of PTL logic circuit performance.
Asynchronous logic design has been established as a competitive alternative to synchronous circuits thanks to the potential for high-speed, low-power, reduced electromagnetic interference, and timing modularity (see J. Sparsø and S. Furber (eds.), Principles of asynchronous circuit design—A systems perspective, Kluwer Academic Publishers, 2001). Asynchronous logic has been developed in the last decade to deal with the challenges posed by the progress of very large-scale integration (VLSI) technologies, together with the increasing number of gates on chip, high density, and GHz operation frequencies. These problems are expected to appear in future high-performance technologies operating at the 10 GHz barrier, due to the increased influence of interconnect on signal delay, uncertainty in the delay of a given gate, and on-chip parameter variations. These factors create difficulties in the design of fast digital systems controlled by a single general clock, due to considerations of delay skew between distant logic blocks, as well as the complexity of design of structures controlled by multiple clocks.
Asynchronous design provides digital systems based on self-timed circuits, which demand no control of a general clock, along with fast communication protocols in which speed depends only on the self delay of the logic gates. The absence of a general clock contributes to low power operation, by eliminating the concentrated power consumption of certain chip areas where numerous transactions occur with arrival of each clock signal.
However, these desirable characteristics usually come at a cost of either silicon area, or speed, or power, and cannot be achieved all at once. Furthermore, asynchronous circuits are typically more complicated than their synchronous counterparts. Although many researchers have sought efficient asynchronous circuit implementations, the disadvantages of current asynchronous logic techniques have not yet been overcome.
There is thus a widely recognized need for, and it would be highly advantageous to have, a digital logic circuit devoid of the above limitations.